Deep peat heat: Microbiome dynamics through space and time at the SPRUCE warming experiment

Christopher W. Schadt, Eric R. Johnston, Laurel A. Kluber and J. Megan Steinweg, Biosciences Division, Oak Ridge National Laboratory, Oak Ridge, TN, Christopher W. Schadt and Paul J. Hanson, Climate Change Sciences Institute, Oak Ridge National Laboratory, Oak Ridge, TN, Christopher W. Schadt, Department of Microbiology, University of Tennessee, Knoxville, TN, Eric R. Johnston, Schneider National, Green Bay, WI, Laurel A. Kluber, Novozymes BioAg, Salem, VA, J. Megan Steinweg, Biology, Roanoke College, Salem, VA, Caitlin C. Petro, Max Kolton and Joel E. Kostka, School of Biology, Georgia Institute of Technology, Atlanta, GA, Malak M. Tfaily, Soil, Water, and Environmental Science, University of Arizona, Tucson, AZ, Rachel M. Wilson and Jeffery P. Chanton, Department of Earth, Ocean and Atmospheric Science, Florida State University, Tallahassee, FL, Paul J. Hanson, Climate Change Science Institute and Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN

Background/Question/Methods
The general consensus is that heterotrophic microbial activity is likely to be stimulated by climate change forcings (warming, atmospheric CO2 enrichment) in freshwater wetlands and peatlands, resulting in increases in greenhouse gas emissions. However, evaluating the response of peat microbial communities to climate change is confounded by their high compositional diversity and complex functional attributes. In addition, past large scale field manipulation experiments in peatlands, and soils more broadly, have too often focused only on shallow responses and microbiome studies have often lacked sufficient depth of sequence coverage. As part of the ongoing DOE SPRUCE whole ecosystem warming experiment, peat communities representing 2015, 2016 and 2018 sample collections (approx. 1, 2 and 4 years after onset of warming respectively) from four different depth increments (10-20, 40-50, 100-125 and 150-175cm) were assessed with shotgun metagenomic sequencing that resulted in >2 terabases of DNA sequence information. These data were further binned into metagenome-assembled genomes (MAGs) and compared with peat and porewater biogeochemical patterns to facilitate our understanding of the microbial communities and processes at both taxonomic and functional levels, through time and in response to warming treatments.
Results/Conclusions
We recovered >400 non-redundant microbial genomes that are estimated to represent ~40% of taxa in the shallowest peat layers and up to 70-90% of taxa in the less diverse intermediate and deep peat layers. Overall community diversity metrics, MAG abundance, and gene content, exhibit strong depth stratification that is consistent with physical and chemical gradients measured in the peat profile. 2015 and 2016 samples showed little effect of warming on whole community diversity patterns or MAG-level abundances, indicating there may be a long response lag to temperature treatments, especially in deeper peat deposits. More recently, changes have been observed at the SPRUCE experiment for various ecosystem properties including: increased CH4 flux, changes in porewater chemistry, increased resin-available nutrients, and changes in plant productivity. These results suggest that the more recently collected deep microbial samples (e.g. 2018 for which analyses are pending) are more likely to show responses to experimental warming, and help explain the observed changes in these various ecosystem properties. We thus expect that application of our genome-resolved approach will be a powerful tool for the evaluation of microbial dynamics and resulting biogeochemical functions in response to climate drivers in peatland ecosystems through space and time.